DIARAD/SOVIM on the International Space Station

The Total Solar Irradiance is the primary source of energy reaching the Earth-atmosphere system. While its variability has been successfully studied over various time scales, its absolute value remains an open issue. Since 1978 diﬀerent radiometers have successively and sometimes simultaneously addressed this problem.
Current measurements from DIARAD/VIRGO, PMO06/VIRGO and ACRIM3 radiometers agree well together but diﬀer from TIM/SORCE by about 4.5 W.m−2 .This diﬀerence is higher than the sum of the claimed individual absolute accuracies of the instruments. In this context the SOLAR payload on the International Space Station embarks the SOVIM package. We give the results of the diﬀerential absolute radiometer (DIARAD) inside SOVIM and discuss its associated uncertainties. The measured TSI value for three days of June 2008 is 1364.50 ± 0.91 W.m−2 for DIARAD/SOVIM left channel and 1364.75 ± 0.91 W.m−2 for the right channel. These values are about 1.2 W.m−2 lower than DIARAD/VIRGO and about 4 W.m−2 higher than TIM/SORCE. Compared to DIARAD/VIRGO, all possible eﬀorts have been made to improve the absolute accuracy. Substantial progress has been made in the aperture area and electrical power measurements. The thermal eﬃciency is based on a full analysis of the optical characterisation. The diﬀerence between the left and right channels measurements is as low as 0.25 W.m−2 which is within the improved accuracy limits.

TSI orbital means from DIARAD/SOVIM left (red dots) and right
(blues crosses) channels. The dispersion of measurements is of the order of 0.13
W.m−2 . The measurements during each orbit are presented for three days of obser-
vations in Figure 4.19. For comparison, DIARAD/VIRGO 3-minute measurements
are also displayed. DIARAD/VIRGO data are shifted by -1.2 W.m−2 .

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